Using molecular genetic approaches, two forms of angiotensin-converting enzyme (ACE) have so far been identified; a pulmonary form and a testicular form. This study utilized both northern blot analysis and in-situ hybridization to examine the expression of ACE mRNA in the rat brain. Northern blot analysis using oligonucleotide probes specific to the testicular or pulmonary forms of ACE demonstrated expression of a 4-5kb mRNA doublet in the rat brain, identical in size to pulmonary ACE mRNA. No expression of the testicular form of ACE mRNA was detected in the brain. In situ hybridization demonstrated that ACE mRNA was expressed in the choroid plexus, caudate putamen and cerebellum. A weak signal was also observed in the hippocampus. This distribution correlated well with the expression of ACE mapped by radioligand binding and autoradiography.